Venous dominance and systemic hemodynamic divergence in a coupled craniopagus circulation

Introduction: Total vertical craniopagus twins provide a unique physiological model of two largely independent cardiovascular systems coupled through a shared intracranial venous compartment. Research question: We investigated whether asymmetric venous drainage could provide a physiologically plausible explanation for persistent inter-twin differences in arterial pressure, central venous pressure, and renal output during staged surgical separation. Materials and methods: We performed a descriptive, longitudinal physiological analysis of systemic hemodynamics, renal output, biochemical variables, transthoracic echocardiography, and angiographic anatomy in female total vertical craniopagus twins undergoing staged neurosurgical separation. Cardiac structure and systolic function were preserved in both twins. Angiography demonstrated largely independent cerebral arterial inflow without clinically significant arterio-arterial cross-circulation and revealed marked asymmetry of the shared intracranial venous drainage system. Results: One of the twins (Twin A) had dominant venous drainage, persistently higher arterial and central venous pressures, and greater urine output, whereas her sister (Twin B) had relative hypotension, tachycardia, and intermittent oliguria. The inter-twin blood pressure gradient changed dynamically during staged surgical modification of shared venous pathways, before subsequent reappearance of the original asymmetric dominance. Ultimately, Twin B died following the final stage of separation. Discussion and conclusions: These observations are consistent with the physiological plausibility that asymmetric venous drainage contributed to the divergent systemic hemodynamic phenotypes observed in the two twins. Within a conceptual Guyton-based physiological framework, progressive modification of the shared venous compartment may have altered venous return conditions and thereby influenced systemic arterial pressure. Although causal inference cannot be established from a single observational case, this study suggests that shared venous architecture may represent an underrecognized contributor to systemic hemodynamic behavior in rare human circulatory systems involving shared venous drainage.